This invention relates to a process for the preparation of semiconductor-grade silicon in which losses of lower-boiling silanes are reduced. More specifically, this invention relates to a process in which these lower-boiling silanes are disproportionated with excess tetrachlorosilane to generate trichlorosilane for easier recovery and containment.
For the purposes of the instant invention, the term "silanes" is used generically to encompass specific compounds including silane (SiH.sub.4), chlorosilane (H.sub.3 SiCl or MCS), dichlorosilane (H.sub.2 SiCl.sub.2 or DCS), trichlorosilane (HSiCl.sub.3 or TCS), and tetrachlorosilane (SiCl.sub.4 or STC). The "lower-boiling silanes" are defined in the instant invention as SiH.sub.4, MCS, and DCS. The term "silane" followed by the chemical formula (SiH.sub.4) will be used to designate silane as a specific compound.
SiH.sub.4, MCS, and DCS have boiling points at atmospheric pressure of -112.degree. C., -31.degree. C., and 8.degree. C., respectively. These silanes are difficult to contain due to their low-boiling points. Superatmospheric pressures and low temperatures are necessary for the recovery and storage of these materials. Losses of these materials during processing or in storage is a significant economic and environmental concern.
The preparation of semiconductor silicon via reductive chemical vapor decomposition/deposition (CVD) is known in the art. Representative examples of the apparatus and method are described in several U.S. patents: U.S. Pat. No. 3,011,877, Schweickert et al., issued Dec. 5, 1961; U.S. Pat. No. 3,099,534, Schweickert et al., issued Jul. 30, 1963; U.S. Pat. No. 3,147,141, Ishizuka, issued Sep. 1, 1964; U.S. Pat. No. 4,150,168, Yatsurugi et al., issued Apr. 17, 1979; U.S. Pat. No. 4,179,530, Koppl et al., issued Dec. 18, 1979; and U.S. Pat. No. 4,311,545, Bugl et al., issued Jan. 19, 1982.
The chemistry of the disproportionation of silicon-containing materials having hydrogen and chlorine ligands is known in the art. The use of solid catalysts or catalysts on a solid support is also known in the art. Jenkner et al., U.S. Pat. No. 3,147,071, issued Sep. 1, 1964, discloses a process for manufacturing dichlorosilane from reaction mixtures such as silane and tetrachlorosilane using activated carbon as a catalyst.
Litteral et al., U.S. Pat. No. 4,113,845, issued Sep. 12, 1978, discloses a process in which dichlorosilane can be prepared from trichlorosilane and silane can be prepared from dichlorosilane. This process utilizes a catalyst which is an ion exchange resin to which is bonded tertiary amino or quaternary ammonium groups.